Chem 471 Part 2: Industrial Electrochemistry
Chem 471 Part 2: Industrial Electrochemistry
2.1 Electrochemical Cells Background 2.2 Batteries 2.3 Fuel Cells 2.4 Corrosion 2.5 Sodium Chloride
2.6 Chloralkali Products (Cl2, NaOH, H2 from aqueous NaCl electrolysis) 2.7 Aluminum Production 2.8 Copper Refining 2.9 Other Electrochemical Industries
Chem 471 Part 2: Industrial Electrochemistry
2.1 Electrochemical Cells - Background why a whole section on electrochemistry? what’s so special about electrochemistry? advantages and disadvantages?
There are different ways to run chemical reactions.
Plan A. Direct Reaction (traditional “shake-and-bake” chemistry) The obvious way to do chemistry is to bring the reactants into direct contact. For example, mix hydrogen and oxygen
Pt catalyst H2 + ½ O2 H2O
The spontaneous reaction to form water proceeds. O atoms (more electronegative) gain electron density, and H atoms lose electron density. Oxygen is reduced and hydrogen is oxidized. Plan B. Electrochemical Reactions A less obvious but important way to do chemistry:
oxidation and reduction reactions occur at different locations electrons are transferred from the chemical being oxidized to the chemical being reduced through an external circuit, usually a metal wire (an electronic conductor) the reactants are separated by an electrolyte solution (an ionic conductor, but not an electronic conductor)
Industrial Electrochemistry: use spontaneous electrochemical reactions to produce electric current do electrical work (batteries and fuel cells, corrosion) apply an electric current to drive electrons in the nonspontaneous direction, to force chemical reactions that are impossible by simply mixing the reactants (electrolysis cells)
Electrochemical Reaction of Oxygen and Hydrogen (spontaneous)
Bubble hydrogen gas over a Pt electrode (why platinum?) and bubble oxygen over another Pt electrode. Dip the electrodes in an aqueous hydrochloric acid solution (the electrolyte, an ionic conductor with mobile H+(aq) and Cl-(aq) ions).Connect the electrodes with a metal wire to conduct electrons externally to generate electric current.
anode (oxidation) cathode (reduction) + + H2(g) 2H (aq) + 2e 2H (aq) + ½ O2(g) + 2e H2O(l)
Overall: H (g) + ½ O (g) H O(l) (n = 2 moles of electrons) 2 2 2 Electrolyse Water to Make Hydrogen and Oxygen (nonspontaneous)
Apply an electric current (using a battery or a dc power supply) to drive the nonspontaneous reaction, stripping electrons from water and “forcing” them onto hydrogen ions.
cathode (reduction) anode (oxidation) + + 2H (aq) + 2e H2(g) H2O(l) 2H (aq) + ½ O2(g) + 2e
Overall: H O(l) H (g) + ½ O (g) 2 2 2 Examples of Anodic (Oxidation) Reactions
Ce3+(aq) Ce4+(aq) + 2e- simple electron transfer
Fe(s) Fe2+(aq) + 2e- anodic dissolution
- - 2Cl (aq) Cl2(g) + 2e gas evolution
2- - Pb(s) + SO4 (aq) PbSO4(s) + 2e phase conversion
+ - 2Al(s) + 3H2O(l) Al2O3(s) + 6H (aq) + 6e oxide formation
+ - CH3OH(l) + H2O(l) CO2(g) + 6H (aq) + 6e fuel oxidation (“cold” combustion)
Examples of Cathodic (Reduction) Reactions
Fe3+(aq) + e- Fe2+(aq) electron transfer
Cu2+(aq) + 2e- Cu(s) metal deposition
- - 2H2O(l) + 2e H2(g) + 2OH (aq) gas evolution
+ - O2(g) + 4H (aq) + 4e 2H2O(l) gas depletion
+ 2- PbO2 (s) +4H (aq) + SO4 (aq) PbSO4(s) + 2H2O(l) phase conversion
- - 2CH2=CHCN + 2H2O(l) + 2e (CH2CH2CN)2 + 2OH (aq) dimerization
Classifying EChem Cells as Spontaneous or Nonspontaneous (remember Chem 231, 232) 1. Using Changes in the Gibbs Free Energy ( G)
From thermodynamics, a chemical reaction is spontaneous at a given temperature and pressure if it decreases the Gibbs free energy.
GT,p < 0 (spontaneous)
GT,p > 0 (nonspontaneous)
o Example H2(g) + ½ O2(g) H2O(l) (at 25 C, 1 bar)
o o o G = Gf (products) Gf (reactants)
o o o = Gf (H2O(l)) Gf (H2(g)) 0.5 Gf (O2(g))
= 237.13 kJ mol-1 0 0 = 237.13 kJ mol-1 Shows the conversion of pure hydrogen and pure oxygen to liquid water is spontaneous at 25 oC and 1 bar. Classifying EChem Cells as Spontaneous or Nonspontaneous (remember Chem 231, 232) 1. Using Changes in the Gibbs Free Energy ( G)
Voltage of an Electrochemical Cell Under reversible conditions (fast electrode reactions, no side reactions) the cell voltage is
Eo = Go/nF = (237.13 kJ mol 1)/(2 (96485 C mol 1)) = 1.229 Volt (positive cell voltage spontaneous reaction)
Electrical Work Also under reversible conditions, the conversion of one mole of H2(g) and one half mole of O2(g) can be used to produce the electrical work
o o 1 we = G = nFE = 237.13 kJ mol
(negative electrical work work done on the surroundings) Classifying EChem Cells as Spontaneous or Nonspontaneous (remember Chem 232) 1. Using Tables of Standard Reduction Potentials
A standard reduction potential is a voltage measuring the relative ease of reducing (adding electrons) to molecules or ions in their standard states (all gases at 1 bar and all dissolved ions at unit activity).
Examples
Fluorine is very easily reduced (F2 is a good oxidizer): - - o o 1/2 F2(g) + e F (aq) ER = +2.87 Volt (25 C)
Lithium ions are difficult to reduce (Li metal is good reducing agent): + - o o Li (aq) + e Li(s) ER = -3.045 Volt (25 C)
Hydrogen is assigned zero standard reduction potential:
+ - o H (aq) + e 1/2 H2(g) ER =0 Volt (definition)
Classifying EChem Cells as Spontaneous or Nonspontaneous (remember Chem 232) o Standard Reduction Potentials at 25 C Li+(aq) + e- → Li(s) - 3.04 V “difficult” Rb+(aq) + e- → Rb(s) - 2.95 K+(aq) + e- → K(s) - 2.92 Ca2+(aq) + 2e- → Ca(s) - 2.76 Na+(aq) + e- → Na(s) - 2.71 Mg2+(aq) + 2e- → Mg(s) - 2.38 Al3+(aq) + 3e- → Al(s) - 1.71 Zn2+(aq) + 2e- → Zn(s) - 0.76 Fe2+(aq) + 2e- → Fe(s) - 0.41 Cd2+(aq) + 2e- → Cd(s) - 0.40 Ni2+(aq) + 2e- → Ni(s) - 0.23 Pb2+(aq) + 2e- → Pb(s) - 0.13 + - 2H (aq) + 2e → H2(g) 0 Cu2+(aq) + 2e- → Cu(s) 0.34 Ag+(aq) + e- → Ag(s) 0.80 “easy”
Classifying EChem Cells as Spontaneous or Nonspontaneous (remember Chem 232) 1. Using Tables of Standard Reduction Potentials
o + - Calculate E for the cell reaction Li(s) + ½ F2(g) = Li (aq) + F (aq) - - o ½ F2(g) + e F (aq) ERed = +2.87 V + - o o Li(s) Li (aq) + e EOx = -ER = +3.04 V ______+ - o Li(s) + ½ F2(g) Li (aq) + F (aq) E = +2.87 + 3.04 V = +5.91 V
Warning! The actual cell voltage will be different if: a) the chemicals are not in their standard states b) the electrode reactions are slow c) there are side reactions, such as + - Li(s) + H2O(l) Li (aq) + OH (aq) + ½ H2(g) 2.2 BATTERIES Electric power is generated by a spontaneous chemical reactions. In a fuel cell, fuels such as hydrogen, methane, methanol, etc. are reacted electrochemically with oxygen.
Advantages • chemical energy is converted directly into electric current, without electrical generators or moving mechanical parts
• from thermodynamic considerations, the full free energy change of the chemical reaction can be converted into electrical work (no heat engine Carnot limitation)
• batteries are the only practical power sources for small portable electronic devices (radios, laptops, flashlights, cell phones, etc.).
BATTERIES
Disadvantages In principle, any spontaneous chemical reaction can be used to directly generate electric current electrochemically. In practice:
• Unwanted side reactions can occur. Aluminum electrodes, for
example, are coated with a thin insulating layer of Al2O3.
• The electrode reactions may be too slow to produce adequate power (energy per unit time) for some applications.
• In competition with combustion engines in cars, trucks, electricity power generating stations, etc., batteries and fuel cells are still too expensive for large-scale use.
Important Battery Systems
Lead-Acid Cell [E = 2.05 V, secondary cell (rechargeable)]
+ 2- - cathode PbO2(s) + 4H + SO4 + 2e 2H2O + PbSO4(s) 1.93 V
2- - anode Pb(s) + SO4 PbSO4(s) + 2e 0.36 V
+ 2- o overall PbO2(s) + Pb(s) + 4H + 2SO4 2PbSO4(s) + 2H2O 2.29 V (E ) (why not 2.05 V?) electrolyte aqueous H2SO4
Uses: car and truck engine ignition emergency power supplies electric vehicles (e.g., fork lifts)
About 150 million lead-acid batteries sold per year, worth about $15 billion (45 % of total battery production). Lead-Acid Cell (secondary cell, rechargeable)
E = 2.05 V The why are car lead-acid batteries rated at 12 V?
Pb PbO2 Lead-Acid Cell (secondary cell, rechargeable)
Good: inexpensive rugged and reliable (proven 150-year-old technology) rechargeable can deliver large pulse currents (400 A) to start engines
Bad: toxic electrodes and electrolyte (lead and sulfuric acid) heavy (low energy density), limiting the use of lead-acid batteries in electric cars and portable electronic devices)
______a) What have lead-acid battery manufacturers done to make the their product more acceptable to consumers? b) How can lead acid batteries be modified to avoid H2SO4 spills or leaks? c) Give 3 reasons why an aluminum-acid battery (if developed!) would be better.
Zinc-Carbon Dry Cell (1.5 V, primary cell, not rechargeable)
+ ! cathode 2MnO2(s) + 2NH4 (aq) + 2e Mn2O3(s) + 2NH3(aq) + H2O 0.6 V complicated! anode Zn(s) Zn2+ + 2e! 0.76 V
2+ ! o overall 2MnO2(s) + Zn(s) + H2O Mn2O3(s) + Zn + 2OH 1.4 V (E )
electrolyte moistened NH4Cl/ZnCl2/MnO2/graphite powder
Uses: Small (but not miniature) portable low-drain electronic devices such as radios, clocks, flashlights, ...
Most popular battery ranked by number produced: 10 billion per year. First commercial dry battery. ______Why are zinc-carbon batteries called dry cells? Why is graphite powder used in the electrolyte? Why is the terminology “carbon-zinc battery” inaccurate? Zinc-Carbon Dry Cell
Good: very cheap nontoxic rugged works in any orientation
Bad: disposable treated as hazardous waste in some jurisdictions
short shelf-life (zinc anode attacked by aqueous NH4Cl) ______Why are gaskets and seals (often overlooked) very important in the construction of dry cells? Why is carbon (not a metal) used as the cathode?
Development of a Better Dry Cell
A case study illustrating how industrial chemists produce new and useful products (and big profits for chemical companies).
The short lifespan of dry cell batteries was bad for sales, consumer confidence and company shareholders.
In the 1950’s a research team led by Lewis Urry at the Eveready
Battery Company improved the dry cell, replacing the NH4Cl/ZnCl2 electrolyte with aqueous KOH. The lifetime of the new battery was longer, but the current was too low.
This problem was solved by using powdered zinc/KOH gel as the anode (why powdered zinc?), leading to the development of the alkaline battery. Alkaline Dry Cell (1.5 V, primary cell, not rechargeable)
! cathode 2MnO2(s) + H2O(l) + 2e Mn2O3(s) + 2OH-(aq) 1.28 V
! anode Zn(s) + 2OH-(aq) ZnO(s) + H2O(l) + 2e 0.15 V
o overall 2MnO2(s) + Zn(s) Mn2O3(s) + ZnO(s) 1.43 V (E ) electrolyte aqueous KOH/zinc powder gel
Uses: Similar to those for zinc-carbon cells
Almost 10 billion sold per year.
More expensive to produce than zinc-carbon cells, but can be sold at higher prices. Alkaline Dry Cell
Advantages over zinc-carbon cells: longer shelf life (up to 10 yr) about three times the capacity for the same size battery
Disadvantages: corrosive KOH electrolyte higher manufacturing costs older models contained mercury to limit cathode side reactions ______Why is it important that zinc-carbon and alkaline cell voltages are the same (1.5 V)? Lithium Ion Cell (3 to 4 V, secondary cell, rechargeable)
+ - cathode CoO2(s) + Li (nonaq) + e LiCoO2(s) anode Li (adsorbed on graphite) Li+(nonaq) + e-
overall Li(adsorbed on C) + CoO2(2) LiCoO2(s) 3 to 4 V
electrolyte LiClO4, LiBF4 or LiPF6 in an organic solvent such as dimethylcarbonate or diethylcarbonate
Uses: laptops, notebooks, cellphones, cameras, electric power tools, electric cars and trucks (all requiring higher energy densities than can be provided by lead-acid or dry cells)
About 2 billion lithium ion cells are made per year. ______
Questions. a) Why use expensive LiClO4, LiBF4 or LiPF6 salts for the electrolyte, instead of much cheaper LiCl? b) Why use expensive organic solvents such as alkylcarbonates, instead of cheaper organic solvents, such as benzene or methanol? c) Why not use water as the solvent?
Lithium Ion Cell
Lithium metal is very good for high energy density batteries because lithium is light (density 0.53 g cm-3, about half the density of liquid water!) and strongly electropositive
Li(s) → Li+(aq) + e− Eo = 3.04 V
But aqueous electrolytes can’t be used. In contact with water, lithium metal spontaneously reduces water:
Li(s) → Li+(aq) + e− 3.04 V
− − e + H2O(l) → ½ H2(g) + OH −0.83 V
+ − Li(s) + H2O(l) → Li (aq) + ½ H2(g) + OH (aq) 2.21 V Lithium Ion Cell Lithium Ion Cell
Good: high energy density (500 to 1000 kJ per kg cell) compared to other cells (about 100 kJ per kg for lead-acid cells) rechargeable (secondary cell) work in any orientation
Bad: expensive compared to lead-acid and dry cells can catch fire* (Why? Li/organic solvent) if overheated or if leaks develop
______*cautionary tales about pushing battery technology to its limits: Galaxy 7 smartphone recall due to lithium ion cells catching fire cost Samsung about $10 billion Boeing 787 Dreamliner fleet grounded for months due to lithium ion battery fires and failures cost Boeing about $1 billion Lithium Ion Cell Lithium ion cell technology is actively investigated in university*, government and industrial labs. Improved electrodes, electrolytes (e.g., solid polymers) cell design are under development. Goals:
higher energy density (especially for electric vehicle applications) faster recharge higher reliability (less degradation, safer to operate)
*Prof. Jeff Dahn
Dal’s “Battery Man” and Herzberg Medalist (Canada’s highest science award)
Less Commonly Used (but important) Battery Systems
Lithium Dry Cells (primary, about 3 V)
Li(s) + MnO2(2) LiMnO2(s) Also called lithium metal cells. (Different from lithium ion cells.) More expensive than other dry cells, but last longer
Silver-Zinc “Button” Cells (primary or secondary, 1.8 V)
Ag2O(s) + Zn(s) 2Ag(s) + ZnO(s) Stable voltage during discharge. Good for miniature devices, such as watches, calculators and heart pacemakers.
Nickel-Cadmium Cells (secondary, about 1.5 V)
2NiO(OH)(s) + Cd(s) + 2H2O(l) 2Ni(OH)2(s) + Cd(OH)2(s) Low maintenance. Used for emergency power supplies. ______Note: Hundreds of other battery systems have been developed. Energy and Power Density of Batteries What’s the difference between energy and power?
Energy density an important design considerations for batteries. Why?
A battery can have a high energy density, but a low power density. How is this possible? Why can this kind of battery still be useful?
Energy Densities lead-acid batteries 0.12 MJ/kg advanced Li ion batteries 1.0 MJ/kg but… gasoline 50 MJ/kg why so high? liquid hydrogen 150 MJ/kg why even higher? ______Is there some way to combine the high energy density of liquid fuels and the advantages of electrochemical cells .… ? .… 2.3 FUEL CELLS ( “Flow” Batteries)
batteries must be replaced (primary cells) or recharged (secondary cells) when the reactants in the anode or cathode compartments are used up
fuel cell: fresh reactants are pumped into the cell and reaction products are pumped out
fuel cells can therefore operate continuously
chemical energy is converted directly into electrical work
fuel-cells are more efficient than heat engines because their performance is not subject to the Carnot heat-engine limitation: :
w/qH = 1 (TC /TH) Hydrogen/Oxygen Fuel Cells
- - cathode ½ O2 + H2O + 2e 2OH 0.40 V
- - anode H2 + 2OH 2H2O + 2e 0.83 V
o overall H2(g) + ½ (g) H2O(l) E = 1.23 V electrolyte aqueous KOH
Uses: limited by high cost and slow cathode reactions, but promising and under active research and development
Other Fuel Cells The hydrogen/air cell operates at about 200 oC (to speed up the electrode kinetics) using phosphoric acid as the electrolyte and hydrogen from steam-reformed natural gas. Cells operating at 600 oC using molten carbonate electrolytes and methane fuel have also been developed. Fuel cells are promising, but still too costly to compete with other power supplies. Hydrogen/Oxygen Fuel Cells (PEM)
Hydrogen/Oxygen Fuel Cells
Application: Space Missions
cost not important here! (why?)
Apollo Service Modules carried
three H2/O2 fuel cells, each with 31 pairs of electrodes in series
30 V, power 500 W to 2000 W
used liquid H2 and O2 onboard for the Service Module main engine
fuel cell waste (H2O) provided drinking water for the crew!
Hydrogen/Oxygen Fuel Cells
o H2(g) + ½ O2(g) H2O(l) 25 C, 1 bar
o o o G = Gf (products) Gf (reactants) o o o = Gf (H2O(l)) Gf (H2(g)) ½ Gf (O2(g)) = 237 kJ mol1 0 0 = 237 kJ mol1
Eo = Go/nF = (237 kJ mol1)/(2 96485 C mol1) = 1.23 Volt
Electrical Work Under standard conditions, the reaction
of one mole H2 and one half mole O2 can produce the electrical work
o o 1 we = G = nFE = 237 kJ mol
OK, but why is this “promising”?
Comparison with a Mechanical Heat Engine
Instead of a fuel cell, burn H2 in direct contact with O2 to produce heat and high-pressure steam to run a turbine heat engine operating
with steam at TH = 500 K and the surroundings at TC = 300 K .
Using standard enthalpies of formation o H2(g) + ½ O2(g) H2O(l) 25 C, 1 bar
o o o heat released qH = H = Hf (products) Hf (reactants) o o o = Hf (H2O(l)) Hf (H2(g)) ½ Hf (O2(g)) = 286 kJ mol1 0 0 = 286 kJ mol1 But from the Second Law of thermodynamics and the Carnot limitation, the maximum mechanical work obtained is
o 1 wmech = H [1 (TC /TH )] = 114 kJ mol
Important Nearly twice as much work can be produced electrically in a fuel cell than in a heat engine (237 kJ vs. 114 kJ). 2.4 CORROSION
The spontaneous electrochemical oxidation of metals.
Example: Rusting of iron and steel to form iron oxides
corrosion can decide the lifetime of oil rigs, pipelines, reactors, … costs about 1 trillion dollars per year many industrial processes employ highly corrosive chemicals equipment failures caused by corrosion can be dangerous if corrosion can be understood, then steps can be taken to fight it CORROSION
Corrosion reactions do not occur directly.
Instead, oxidation half-reactions such as
Fe(s) Fe2+ + 2e 0.44 V (Eo) and reduction half-reactions such as
+ ½ O2 + 2H + 2e H2O (acidic solutions) 1.23 V
+ 2H + 2e H2 (acidic solutions) 0.00 V
½ O2 + H2O + 2e 2OH (alkaline solutions) 0.40 V occur electrochemically at different locations on the metal. Galvanic Corrosion When two different metals are brought into contact in the presence of air and moisture, the metal with the less positive reduction potential (easier to oxidize) will act as the anode and dissolve, while the other metal will act as the cathode. Galvanic corrosion can be very rapid.
Crevice Corrosion If a crack or a crevice develops in a metal object, the outer part of the crevice (exposed to air) acts as the cathode and the inner part acts as the anode (metal oxidation and dissolution). Once crevice corrosion begins, the depth of the crevice will increase which can result is holes and structural failure. Corrosion Caused by Paint Scratches Paint or other surface coatings can protect metals from corrosion. But if the coating is scratched, the exposed metal acts as a cathode
+ example: 1/2 O2 + 2H + 2e H2O and the nearby painted metal acts as the anode and dissolves
example: Fe(s) Fe2+ + 2e
Causes paint to lift and peel along the scratch, leading to further corrosion.
Fix paint scratches! Corrosion Control
avoid air, water, and other corrosive chemicals (usually impractical!)
coat metal with a protective layer (paint, a more resistant metal, oil)
repair holes or scratches in the protective layer
avoid contact with less electropositive metals (galvanic corrosion)
coat or connect the metal to be protected to a more electropositive metal that acts as sacrificial anode which corrodes first
example: galvanized steel (protective zinc coating)
Zn Zn2+ + 2e (anode) 0.76 V + 1/2 O2 + 2H + 2e H2O (cathode) 1.23 V
Up next: Electrolytic Chemical Industries
Electrolysis Use an applied voltage to pump electrons into the cathode and pull them out of the anode: forcing a nonspontaneous electrochemical reaction to occur
Three most important electrolytic industries:
1. Brine (Aqueous NaCl) Electrolysis (chloralkali industry)
+ + 2Na (aq) + 2Cl (aq) + 2H2O 2Na (aq) + 2OH (aq) + Cl2 + H2
2. Aluminum Production
2Al2O3 + 3C 4Al + 3CO2
3. Copper and Zinc Electro-Refining
2+ 2 2 Cu (aq) + SO4 (aq) + 2e Cu(s) + SO4 (aq) 2+ 2 2 Zn (aq) + SO4 (aq) + 2e Zn(s) + SO4 (aq) 2.5 SODIUM CHLORIDE (“Salt”)
NaCl is not included in most “top 50" lists of industrial chemicals.
Why? Most industrial NaCl is “captive” (produced and used by the same manufacturer to produce other chemicals, not sold). Also, NaCl occurs naturally in very pure form (rock salt, typically 98 to 99% pure NaCl), so little or no chemistry is involved in salt production.
But in terms of tonnage, NaCl ranks near sulfuric acid (#1 on the lists).
Uses of NaCl 45% chloralkali production (Cl2, NaOH, H2) 20% other industrial chemical manufacturing 25% ice control on roads 5% food products 5% miscellaneous
SODIUM CHLORIDE (“Salt”)
Vast amounts of NaCl are available in seawater and beds of rock salt.
NaCl production
1. from brine (50 %)
2. underground mining (30 %)
3. seawater evaporation (20 %)
Sifto Salt Mine under Lake Huron, near Goderich, Ontario NaCl Production 1. Brine Water is pumped into drilled salt beds. A saturated solution containing ~25 % NaCl (and other dissolved salts) is pumped out and 2+ 2+ 3+ treated with Na2CO3 to precipitate Ca , Mg and Fe . The purified brine is used directly or evaporated to precipitate 99.8% pure NaCl.
2. Mining Mines are dug into deposits of rock salt (typically 98 to 99% pure NaCl). Clay, sand and other solid impurities are removed by sieving or gravitational separation. This product is suitable for salting roads. Further purification required for NaCl used in chemical production is carried out by dissolving the solid NaCl in water and 2+ 2+ 3+ using Ca(OH)2 or Na2CO3 to precipitate Ca , Mg and Fe .
3. Seawater Evaporation First, Mg2+ and most Ca2+ are removed as described above, then water is removed by evaporation, usually in large, shallow solar ponds. Precipitated NaCl (typically 99.8% NaCl) suitable for electrolysis or food products. 2.6 CHLORALKALI PROCESSES
Aqueous NaCl solutions are electrolyzed to make Cl2, NaOH and H2.
Significance
70% of all industrial chemical products use Cl2 and/or NaOH in one or more synthesis steps
this is the largest electrochemical industry (75 million tonnes Cl2, 80 million tonnes of NaOH per year)
illustrating the competitive nature of process economics, pollution control and safety considerations, three different chloralkali processes are used: 1. diaphragm cells 2. membrane cells 3. mercury cells
Chloralkali Industry
Aqueous NaCl solutions are electrolyzed to make Cl2, NaOH and H2.
why not use natural deposits of Cl2 and NaOH?
why are Cl2 and NaOH electrochemically synthesized, not prepared by direct chemical reactions, such as
Ca(OH)2(aq) + Na2CO3(s) → CaCO3(s) + 2 NaOH(aq)
HCl(aq) + MnO2(s) → MnCl2(aq) + 2 H2O(l) + Cl2(g)
why is Cl so stable compared to Cl2?
why are mercury chloralkali cells banned in many countries 1. DIAPHRAGM Chloralkali Cells simple but effective chlor-alkali technology Anode plates are mounted vertically and parallel to one another. Flat, hollow steel mesh cathodes fit between the anode plates in a “toast- rack” arrangement. A diaphragm consisting of a mat of asbestos fibers outside the cathodes provides a physical barrier between the anode (Cl2 produced) and the cathode (H2 and OH produced) solutions. anode 2Cl (aq) Cl2(g) + 2e Diaphragm Cell (side view) cathode 2 H2O(l) + 2e H2(g) + 2OH (aq) DIAPHRAGM Chloralkali Cells
typical diaphragm cell:
dimensions 3 m by 2 m by 2 m
1,200 amp current
voltage 3.2 V to 3.8 V
3.5 tonnes Cl2 per day DIAPHRAGM Chloralkali Cells
Advantages: simple low-cost cell design cells are easy to operate
Disadvantages: chloride contamination Ions are free to diffuse through the pores of the diaphragm, a simple physical barrier, so the NaOH solution is contaminated with Cl ions from the brine on the other side of the diaphragm. The solution leaving the cathode chambers is 15% NaCl and 12% NaOH. The NaCl content must be reduced.
low NaOH concentration After leaving the cells, evaporation is used to concentrate the NaOH from to 50% (the usual commercial product), adding to the cost of the diaphragm process. NaCl has a low solubility in concentrated NaOH solutions, so most of the NaCl precipitates out, reducing the NaCl concentration to about 1%. DIAPHRAGM Chloralkali Cells
Disadvantages:
electrical resistance of the diaphragm The diaphragm partially blocks the ionic current flowing between the anode and cathode. To overcome this resistance for useful production rates, a voltage difference of 3.2 to 3.8 V is required, considerably higher than 2.2 V required for a cell with no internal resistance. And the brine feed must be purified to avoid precipitation of Mg(OH)2 and Ca(OH)2 which would clog the diaphragms and further raise their resistance.
diaphragm lifetime Due to clogging, asbestos diaphragms must be replaced every few months. Requires cell dismantling.
asbestos Requires special handling and disposal procedures.
2. MEMBRANE Chloralkali Cells
ingenious materials science – membranes that selectively transport Na+ ions !
The problems with asbestos diaphragms prompted research to develop membranes that transport Na+ ions, but block the undesirable flow of Cl and OH anions between cathode and anode compartments.
Membrane Chloralkali Cell
Diagram not to scale!
In practice, the membranes are very thin (about 0.2 mm) and the electrodes are very close together (4 mm apart). Why? MEMBRANE Chloralkali Cells Cell Room:
MEMBRANE Chloralkali Cells
ingenious materials science – membranes that selectively transport Na+ ions !
Chloralkali cell membranes are thin sheets of perfluorinated polyethylene with side chains terminating in sulfonate groups (tradename “Nafion”). The membranes contain microscopic “pockets” (about 2 nm diameter) connected and lined with sulfonate groups. Na+ ions are transported through the membrane by being passed from one sulfonate group to the next, making the membrane a cation conductor. MEMBRANE Chloralkali Cells
Advantages: very pure NaOH solutions are produced (< 50 ppm Cl) less power is required than for the mercury process no mercury or asbestos is used.
Disadvantages: highly purified brine is required to avoid fouling the membranes
with precipitated Mg(OH)2) and other impurities because the membranes are not perfectly cation selective (small amounts of OH leakage), the NaOH concentration is limited to a maximum of about 30 %, requiring some water evaporation
the Cl2 produced is contaminated with some oxygen. 3. MERCURY Chloralkali Process
Ingenious technology!
Anode: 2Cl (aq) Cl2(g) + 2e
Cathode: Na+(aq) + Hg(l) + e NaHg(l amalgam)
Liquid mercury serves as the cathode, also absorbs sodium metal, forming a liquid metal sodium + mercury solution (an amalgam).
The Na-Hg amalgam (typically 0.5% Na by weight), after leaving the cell, is treated with water. Produces concentrated (~ 50 %) aqueous NaOH of very high purity (< 30 ppm chloride).
+ 2NaHg(l) + 2H2O(l) 2Hg (l) + 2Na (aq) + 2OH (aq) + H2(g)
The mercury is recycled to the cell. MERCURY Chloralkali Cells
A typical cell consists of thin (about 3-mm thick) layer of mercury in the bottom of a shallow steel trough which slopes slightly to promote the flow of mercury. Horizontal anodes (adjustable in height) are fitted in the cell lid together with slits through which Cl2 gas is drawn off.
Mercury chloralkali plants have cell rooms larger than a football field, use 100 MW of power and 250,000 amperes of current to produce 250,000 tons of chlorine per year. MERCURY Chloralkali Cells
Graphite Anodes were used for many years. But these anodes are not completely inert and the electrode kinetics are slow, requiring a 0.5 V overpotential to speed up the oxidation of Cl ions.
Dimensionally Stable Anodes (DSA) After considerable R&D, the anodes are now titanium mesh coated with RuO2 and smaller amounts of other metal oxides (Co3O2 or PdO2) to act as catalysts. These anodes are called DSA’s because they are resistant to wet chlorine and last 5 to 10 years. Important: The catalysts reduce overpotentials to < 0.04 volt. DSA’s are also used in diaphragm and membrane cells. MERCURY Chloralkali Cells
Cell Voltage
The standard voltage for the mercury cell (3.25 V) 2Cl(aq) Cl2(g) + 2e 1.36 V Na+(aq) + Hg(R) + e NaHg(l amalgam) 1.89 V 3.25 V is less favorable (more negative than for the simple cell (2.20 V) 2Cl (aq) Cl2(g) + 2e 1.36 V 2H2O(l) + 2e H2(g) + 2OH (aq) 0.84 V 2.20 V As a result, mercury chloralkali cells require a larger voltage and energy costs are higher. This disadvantage is offset by the high purity of the products and the high concentration of aqueous NaOH produced (~50 weight %, suitable for direct sale). For adequate production rates, about 4.5 volts is applied to each mercury cell. Mercury Chloralkali Cells
Advantages: very pure Cl2 gas NaOH solutions (< 50 ppm Cl ) are produced
50 % NaOH solution produced directly (no H2O evap. needed) simple cell design, low maintenance brine purification to remove Mg2+ and Ca2+ ions less rigorous than for diaphragm and membrane cells
Disadvantages: higher voltage and energy cost compared to diaphragm and membrane chloralkali cells high costs of mercury release avoidance and monitoring mercury cells are banned in Japan and Europe Chloralkali Industry
Three competing technologies are available.
1. diaphragm cells 2. membrane cells 3. mercury cells
Consensus?
The overall economics of mercury and diaphragm cells are similar.
But membrane cells are about 10% less costly to operate and are gradually becoming the choralkali technology of choice.
2.7 ALUMINUM
2nd-largest electrolytic industry (after chloralkali production)
about 60 million tonnes of aluminum produced per year
aluminum metal is widely used because it is remarkably light strong malleable (can be drawn and stamped without cracking) corrosion-resistant ( why?)
essential for aircraft construction
main alternative to copper for electrically conducting wire
aluminum is the most important nonferrous metal (more important than copper, lead or zinc) ALUMINUM Aluminum Ore The most important aluminum ore is bauxite, a sedimentary rock containing hydrated aluminum oxides, such as Al(OH)3 and AlO(OH), together with iron oxides, silicates, clay and other impurities.
Bayer Process for Alumina (Al2O3) Production from Bauxite Crushed bauxite ore and aqueous NaOH are heated in a pressure vessel to about 150 oC and 20 bar ( why use high pressure?). Al3+ ions dissolve, but iron oxide, silicates and other impurities are insoluble.
After filtration and cooling, pure Al(OH)3 (gibbsite) is precipitated.
Gibbsite is heated to 1200 oC to drive off water, producing high purity aluminum oxide: Al(OH)3(s) = Al2O3(s) + H2O(g)
More than 90% of Al2O3 production is used to make aluminum. Most of the rest is used to make refractory bricks, glass and abrasives. ALUMINUM Aluminum Production
Direct Chemical Reaction? No. In principle, aluminum can be obtained from the oxide by reaction with carbon (as used for iron production):
2Al2O3 + 3C 4Al + 3CO2
But this reaction is thermodynamically unfavorable. Aluminum “likes” oxygen too much to give it up to carbon.
Small amounts of aluminum were first prepared by reacting anhydrous aluminum chloride with potassium (a very strong reducing agent):
AlCl3 + 3K = Al + KC1
Due to the high cost of the reagents and the low yields, aluminum produced by this reaction was more expensive than gold or platinum! ALUMINUM Aluminum Production
Electrolysis of aqueous Al3+ solutions? No. Electrochemical reduction of aqueous Al3+ ions might seem to be a possibility. But water is reduced instead:
Al3+(aq) + 3e Al(s) 1.71 V
2H2O(l) + 2e H2(g) + 2OH (aq) 0.83 V (more favorable)
3+ Electrolysis of Al ions in nonaqueous molten Al2O3? No. The melt is nonconducting nonionic liquid. Also, alumina is a refractory material used to make firebricks to line furnaces. It melts at 2020 oC, a temperature too high for economical industrial processing.
ALUMINUM Aluminum Production
Hall-Heroult Process – finally the breakthrough
Electrolysis of Al2O3 dissolved in molten cryolite (Na3AlF6). The melts contain 7 to 12% aluminum oxide, near the aluminum oxide + o cryolite eutectic (minimum melting point) at 10.5% Al2O3 and 960 C.
Al2O3 + NaAlF3 melts are ionic conductors
eutectic depression of the freezing point (from 2020 oC for pure o Al2O3 to 1000 C) makes the process economical
Cryolite (sodium hexfluoroaluminate) is a rare mineral. There is only one mine with commercial deposits (Ivittut, Greenland). Today most cryolite is synthesized using
6NaOH + 2Al2O3 + 6HF = 2Na3AlF6 + 6H2O ALUMINUM Aluminum Production
Hall-Heroult Electrolysis Cells
The cells are constructed using large steel tanks (9 m 3 m, 1 m deep) open at the top and lined with alumina refractory bricks then carbon.
The base of the tank is lined with carbon blocks inlaid with steel bars to improve the electrical conductivity.
Molten aluminum, which is slightly denser than the alumina + cryolite melt, collects at the bottom of the tank and acts as the cathode.
The anodes, also carbon, are lowered into the tank from above at a rate of about 2 cm per day to compensate for the carbon lost by reaction.
To reduce heat loss, a crust of solid alumina + cryolite is allowed to form at the top of the exposed melt. . ALUMINUM Aluminum Production
Hall-Heroult Electrolysis Cells
About 4.5 volt is applied to each cell, which is significantly higher than the standard cell voltage owing to slow anode kinetics and the electrical resistance of the melt and carbon electrodes.
To reduce pollution, the fluoride-containing gases leaving the top of the cells pass over beds of powdered alumina adsorbents. ALUMINUM Aluminum Production
Hall-Heroult Process Electrode Reactions. Ideally:
cathode Al3+(in the melt) + 3e Al(liquid)
2 anode 2O O2 + 4e
But it’s impossible to find economical electrodes that resist attack by oxygen at cell temperatures (~1000 oC). In practice, the cells operate with consumable carbon anodes. The overall cell reaction is
2Al2O3 + 3C 4Al + 3CO2
Although the anodes must be replaced, formation of CO2 as a reaction product (instead of O2) changes the applied cell voltage from about 5.5 V to 4.5 V, significantly reducing energy costs. ALUMINUM
Aluminum production is energy-intensive, requiring 14 to 18 kW hr per tonne of aluminum (compared to 3 kW hr per tonne of chlorine produced by electrolysis). Aluminum production consumes about 5 % if the electricity generated in North America. There are ten aluminum production plants in Canada. Why are nine located in Quebec?
The formation of CO2 at the anodes significantly reduces the energy costs for aluminum production. Why ?
Pure aluminum is generally too soft for many applications (vehicle and machine parts, structural beams, cans, foil, …) How is it hardened?
Raw aluminum is manufactured in the form of large multi-tonne billets. How are aluminum billets economically shaped to form consumer products? 2.8 COPPER
2nd-most important nonferrous metal (after aluminum)
about 20 million tonnes of copper are produced per year
excellent electronic and heat conductor
resists corrosion
main uses building construction (pipe, wiring, roofing, etc.,) electronics (wiring, circuit boards, electric motors, etc.,) consumer products machine parts
important copper alloys: brass (copper + zinc) bronze (copper + tin) COPPER
Copper resists corrosion, but architectural copper turns green. Why?
COPPER Copper Ores
The main copper ores are the sulfide minerals chalcopyrite (CuFeS2), bornite (Cu5FeS4), covellite (CuS) and chalcocite (Cu2S) obtained from large open-pit mines. Chile, Peru and U.S. are the largest producers.
open pit copper mine (Bingham, Utah)
challenges: 1) typical copper ores contains only 0.6 % Cu 2) copper is not easily extracted from sulfides
COPPER Copper Extraction
Step 1 froth flotation to enrich Cu content from 0.6% to 20% Finely-ground ore is mixed with water, oil and surfactants in large tanks. Compressed air is injected to form streams of bubbles. Oil-coated grains of copper ore preferentially stick to the bubbles and are floated to the top of the tanks where they are skimmed off and collected. Clay, sand and other impurities sink to the bottom of the tanks as sludge.
Step 2 smelting to remove iron and silicate impurities as slag Enriched copper ore is dried and melted together with
limestone (CaCO3) and silica (SiO2). Iron impurities dissolve in the molten Ca/Si slag. Liquid Cu2S, insoluble in the slag and denser, sinks to the bottom of the crucibles and is tapped off. COPPER Copper Extraction
Step 3 roasting Cu2S to produce Cu2O
Hot Cu2S from the smelter is reacted with oxygen from air
Cu2S + 3/2 O2 = Cu2O + SO2 to convert cuprous sulfide to cuprous oxide. Pollution control is important here. Why? What byproducts can be obtained?
Step 4 heating Cu2O to produce blister copper (raw copper) Cuprous oxide is easily decomposed by heating to form “blister” copper (named after it’s appearance). But the purity of the copper is too low for many applications.
Step 5 electrorefining of blister copper to produce 99.9% pure Cu COPPER Copper Electrorefining
To purify copper by electrorefining, impure blister copper is cast to form thick anodes for oxidation in aqueous copper sulfate/sulfuric acid solutions. The cathodes are thin sheets of previously purified copper.
anode Cu(s, impure anode) Cu2+(aq) Eo = 0.34 V
cathode Cu2+(aq) Cu(s, pure cathode) Eo = 0.34 V ______overall Cu(s, impure anode) Cu(s, pure cathode)
Impurities are “left behind” when the aqueous Cu2+ ions produced by oxidation at the anode are reduced and deposit on the cathode.
COPPER Copper Electrorefining Cells
The cell design is very simple: vertical parallel plate electrodes in open tanks lined with rubber or plastic. The electrolyte is an aqueous solution containing about 5% dissolved CuSO4 + 15% H2SO4.
The applied cell voltage is very low ( Why?), only about 0.4 V COPPER Copper Electrorefining Cells
Ingenious electrochemistry! at the anode Copper and less-noble metals (such as iron, zinc, nickel) are oxidized to form aqueous metal ions.
Nobler metals (such as silver, gold, selenium tellurium) do not dissolve and sink to the bottom of the tank as sludge. cathode Dissolved Cu2+ ions migrate through the electrolyte and are reduced at the cathode as 99.9 % pure copper. Less noble metal ions (Fe2+, Zn2+, Ni2+) remain in solution.
The sludge is used to improve the overall economics. How? 2.9 Other Industrial Electrochemical Industries
hydrometallurgical production of copper and zinc by electrochemical extraction from ore leachate (eliminates high
temperature smelting and roasting steps, no SO2 produced)
chlorate (e.g., NaClO3) and perchlorate (e.g., NaClO4)
F2 from electrolysis of molten KF + HF
water electrolysis (source of ultrapure H2 and O2)
magnesium and sodium from electrolysis of molten chloride salts
organic electrosyntheses (e.g., adiponitrile for nylon manufacture)
electroplating
electromachining (e.g., “drilling” square holes!)